Mobile data transmission and data services are constantly making progress. With the increasing penetration of such services, also the transmitted data volume is continuously increasing. Given the constraints of limited bandwidth for transmission, data throughput is an important aspect for users as well as network operators.
In high speed downlink packet access, HSDPA, MIMO (Multiple Input Multiple Output) is a new feature that is becoming more common in practical networks. It relies on plural, e.g. two transmit and reception antennas to be able to transmit more data over the same bandwidth. One problem with MIMO is that older terminals that do not use MIMO have no knowledge of the second transmit antenna, and therefore the usage of MIMO in the network causes extra interference. This problem also affects terminals that support MIMO, but for some reason are not in MIMO mode (e.g. MIMO mode is disabled or switched off for reasons of power saving or network configuration). The problem may also occur in connection with terminals that only have one reception antenna and therefore do not support MIMO (but are new enough so that MIMO can be taken into account in their design).
Such problem has been recognized by operators and network vendors, and measures have been taken to minimize the effect on non-MIMO terminals. These measures include for example turning the second transmit antenna off when there are no MIMO terminals in the network, and scheduling transmissions so that large amounts of data are not sent from the second antenna when non-MIMO terminals are receiving data. In such cases, data is sent only via the first antenna, thereby reducing data throughput.
However, an unsolvable problem resides in the MIMO pilot channel as a particular example of a control channel. A HSDPA network that supports MIMO transmits two pilot channels: a primary common pilot channel, P-CPICH, on the primary antenna for all users, and a secondary common pilot channel, S-CPICH, on the second antenna for MIMO users only. The S-CPICH channel contains only a known pilot pattern and it is used for channel estimation in the receiver. Since the pilot channel contains a known pattern of bits, if the channelization code used for conveying/encoding the pilot channel is known, the S-CPICH can be cancelled and the effect on non-MIMO terminals can be nullified. Unfortunately, non-MIMO terminals have no knowledge of the channelization code or even if the channel is used at all.
Since MIMO is just being taken into use the problem has not existed before and there are no solutions to such particular problem. Pilot channels (control channels) have been cancelled before, but in all those cases, all relevant information about those channels has been known (i.e. was signaled from the network to the receiver).
Also, previous efforts in standardization in connection with MIMO and HSDPA had in focus that information on the control channel, such as S-CPICH information, should be transmitted to non-MIMO terminals. However, this does not seem to represent an optimum solution in view of at least an increased signaling.
Thus, there is still a need to further improve such systems such as MIMO systems.